Complete the game. Thats how you go back. Although, you have to start from the beginning, you get an added bonus - You get to keep everything you got from your last run through. Hilts, crystals, costumes, holocrons, and, powers. After I started over the first time, I was able to one shot everything up to Raxus Prime (Except for Marek's daddy, Kota, and Desolous.)